This included extension (lemontrail_exporter.zip) allows you to build and export custom LemonTrail data from mesh objects in Blender. 

Installation

To install the LemonTrail Exporter extension:

1. extract the LemontrailPackage.zip and find the file "lemontrail_exporter.zip".
2. Open Blender and go to Edit > Preferences.
3. In the Preferences window, go to the Add-ons tab.
4. Click the V arrow in the top right, thenInstall From Disk... and select the lemontrail_exporter.zip file.

Using the Extension

Once the LemonTrail Exporter extension is installed and enabled, you can access its features from the LemonTrail panel of the 3D Viewport sidebar.

1. Invert Radius On/Off

This is useful for instances where you want your trail's normals to face inwards instead of outwards, usefule for all kinds of effects. To invert the selected vertices:

1. Select the mesh object you want to modify.
2. Enter Edit Mode by pressing Tab,
3. Select your desired faces.
4. Go to the LemonTrail tab in the 3D Viewport sidebar.
5. Click Invert On to invert the radius of the selected vertices.
6. If you need to revert the inversion, click Invert Off.

2. Set and Clear Trail

To set an edge to emit trail geometry:

1. Select the mesh object you want to modify.
2. Enter Edit Mode by pressing Tab.
3. Select your desired Edges.
4. Go to the LemonTrail tab in the 3D Viewport sidebar.
5. Click Set Trail to mark the selected vertices for trail spawning.
6. Click Clear Trail to remove it.

3. Toggle Invert Preview

To toggle the visibility of the InvertRadius vertices in the instance that you've forgotten if you've inverted it already:

1. Select the mesh object you want to preview.
2. Go to the LemonTrail tab in the 3D Viewport sidebar.
3. Click Toggle Invert Preview to preview which vertices are set to inverted. They will show up red.
4. Control + Tab to exit this mode. (this will be improved in the future)

4. Export LemonTrail

To export the selected LemonTrail for use in Gamemaker:

1. Select the mesh object(s) you want to export. You can select multiple at a time.
2. Go to the LemonTrail tab in the 3D Viewport sidebar.
3. Click Export LemonTrail.
4. Choose the file location and name, then click Export.

5. Controlling your profile

To control how your trail spawns, you need to define custom "normals", which is far more difficult to do in Blender than one would like. So I created a simple workaround.

The blue edges are trail spawners, each of the lines coming off of it control the normal of each vertex (the direction in which to push and pull the trail along it's profile), and the length that the adjacent vertices will adhere to when creating the trail. the longer your lines are coming off of the trail edges, the longer the trail will be at that edge. If you want your trail to come to a closed point, just make sure your extruded edges join up with each other.  

I'll go over this in more detail very soon, but a good rule of thumb is to extrude your edges, then merge them into the center. This will give you a clean, fully closing mesh trail.